1. Field of the Invention
This invention relates to the processing of three-dimensional images generated by imaging devices, such as computer tomography (CT) and magnetic resonance (MR) imaging systems, and more specifically, to three-dimensional digital subtraction angiography.
2. Background of the Invention
Diagnostic imaging, and in particular medical diagnostic imaging, is generally provided by CT and MR systems, as well as those using positron emission tomography (PET), and other techniques. One particularly desirable use for such systems is the imaging of blood vessels in a patient, i.e. vascular imaging. Vascular imaging methods include two-dimensional (2D) techniques, as well as reconstruction of three-dimensional (3D) images from 2D image data acquired from such diagnostic imaging systems. In CT medical diagnosis, for example, 3D reconstruction of computed tomograms is particularly useful for visualizing blood vessels.
Conventional (2D) angiography is considered the most accurate technique for medical diagnosis of vascular structures and remains the standard against which other methods are compared. However, conventional angiography is an invasive technique and therefor presents a certain amount of risk. Accurate evaluation of the vascular system with noninvasive techniques remains an important goal. Thus, duplex ultrasound is often used for evaluation of blood flow in carotid arteries. Magnetic resonance angiography is also used for detailed evaluation of the vascular system. However, both of these techniques have limitations and alternative noninvasive approaches continue to be investigated.
Spiral computed tomography (CT) is a relatively new approach to CT that allows continuous data collection while a subject is advanced through the CT gantry. This provides an uninterrupted volume of data. From this data, multiple contiguous or overlapping sections of arbitrary thickness can be reconstructed. Spiral CT permits acquisition of a large volume of data in seconds. With spiral CT angiography (CTA), vascular structures can be selectively visualized by choosing an appropriate delay after injection of a contrast material. This gives excellent visualization of vessel lumina, stenoses, and lesions. The acquired data can then be displayed using 3D visualization techniques (e.g., volume-rendering, maximum intensity projection (MIP), and shaded surface display) to give an image of the vasculature. In contrast to conventional angiography, CTA is three-dimensional, thus giving the viewer more freedom to see the vasculature from different viewpoints. In view of this viewpoint freedom, a key task in CTA is to suppress as much data representative of bone from the CT data as possible, without suppression of data representative of the vasculature.
Maximum Intensity Projection (MIP) is a commonly used technique for displaying 3D vascular image data. MIP relies on blood in the vessels having a higher display pixel intensity than other organs in the imaged anatomy. This relationship, however, may not be obtainable for certain types of tissue, depending upon the modality of the imaging device. For example, in CT imaging, wherein a contrast agent is injected into the blood vessels to enhance their x-ray visibility, the coefficient of absorption of the bones, and hence their pixel intensity on the CT display, tends to be of a higher value (in Houndsfield units) than that of the contrast enhanced blood vessels. An analogous problem exists with MR and emission imaging systems. Thus, in many instances, in order to provide an unobstructed view of the vasculature, structures having pixel intensity values similar to or higher than that of the blood vessels must be removed. In accordance with current 3D imaging techniques, this removal is accomplished by post-imaging editing of the acquired image data. The 3D editing task is particularly important for the suppression of bone and other dense tissues, such as calcification, which appear in the acquired data and must be removed/suppressed in order to visualize the data representative of the vessels of interest without obstruction.
Current 3D CT imaging techniques, such as spiral CT angiography (CTA), commonly employ either automatic or semi-automatic post-imaging editing techniques to remove/suppress non-vascular image data. Editing techniques are particularly reliable for removing those portions of the image data which are not of interest. Although CT angiography is a valuable diagnostic tool for vascular imaging, there are several problems. Firstly, as noted above, bone and other dense tissues usually obscure the opacified (contrast enhanced) blood vessels, and secondly, high doses of contrast agent are needed in order to reach the blood vessels in the imaged area. Furthermore, the editing techniques of the prior art solutions require varying amounts of user interaction in order to edit the data and remove unwanted portions from the region of interest.
More specifically, one reliable prior art method for removing undesirable image structures utilizes manual editing methods. These methods employ an expert who manually draws outlines of the structures to be removed on every image slice, using careful, hand-directed cursor manipulations. A major disadvantage of such methods is that manual editing is a repetitive, and therefore very time-consuming process. Since 3D CT imaging develops a large number of image slices to be edited, manual editing consumes expensive machine and operator time, and is therefore undesirable.
Numerous other interactive schemes and methods have been proposed in the prior art to help users edit images more efficiently. One example of such a method is described in an article entitled AN IMAGE EDITOR FOR A 3D-CT RECONSTRUCTION SYSTEM by Jay Ezrielev et al. published in Proceedings of Medical Imaging IV, Image Processing, Newport Beach, 1990, Vol. 1233. An image editing system is described which utilizes intelligent and semi-automatic methods, such as thresholding operations, for removing simple objects from the image data set.
Another CT image editing method is described in an article published in IEEE Computer Graphics and Applications, November 1991, entitled EDITING TOOLS FOR 3D MEDICAL IMAGING by Derek R. Ney et al. An editing method is described which is patterned after a paint and draw program. The editing method lets the user interactively create shapes manually, which are then used to define volumes of interest to be edited in the image data set.
More recently, a faster and more user-friendly interactive editing method has been developed, as described in U.S. patent application Ser. No. 08/055,614, filed Apr. 30, 1993. This editing method makes use of the ability to view several consecutive image slices as one superimposed image, which can then be visualized and manually edited. Thereafter, the editing of the superimposed image is automatically applied to the individual image slices. In a further improvement to this prior art editing method, an outline of the regions to be removed from the CT images can be carried out automatically by a thresholding or comparison operation.
Although such manual and semi-automatic editing techniques greatly improve the visualization of the blood vessels in view of bone and other dense tissues, it is quite unlikely that any of these editing techniques will become completely automatic or will succeed to remove calcification.
Furthermore, most editing approaches introduce the potential for loss or distortion of information when a connectivity algorithm uses too low a threshold to exclude bones physically touched by the vessels, or when the editing procedure attempts to suppress a vessel wall calcification that contacts the vessel lumen. Moreover, editing often requires an average 15-30 minutes of operator time and, in some cases, such as the petrous carotid region, may even take hours. Finally, the quality of the editing is subjective, and depends upon the experiences of the editor. For example, to successfully separate intramural calcium from intraluminal contrast material requires reference to the unenhanced images.
Additionally, with conventional CT angiography high levels of contrast media are introduced intravenously to the patient in order to opacify the blood vessels. Such high levels of contrast agent are undesirable in that patients with heart, and/or kidney problems are at particular risk due to the introduction of the contrast agent.
It is desirable to provide a method and apparatus which would improve the quality of the vascular images produced using 3D imaging techniques.
It is also desirable that the improvement result by an automatic, rather than semi-automatic or manual processing of the acquired data set.
Furthermore, in CT applications, it is desirable to reduce the concentration of the contrast medium needed to opacify the blood vessels in the region of interest.
It is noted that one technique currently used for 2D vascular imaging uses image subtraction techniques, and is commonly referred to as digital subtraction angiography (DSA). Current thinking does not suggest an extension of such 2D DSA techniques to a 3D imaging system since 2D imaging has a relatively fast image acquisition time period, on the order of a fraction of a second to several seconds, while 3D imaging techniques require approximately one minute or even more to acquire each image. Since DSA techniques require that two successive images be acquired (one with a contrast medium and one without), if image data is acquired using a 3D imaging technique there is a much greater chance of significant patient movement, thereby causing the subtracted image to have substantial movement-related artifacts. Additionally, 2D DSA techniques are generally considered as invasive procedures, while 3D imaging is considered a minimally-invasive imaging procedure. Thus, the extension of 2D DSA techniques to 3D imaging is not considered feasible.
It would be desirable to find a way to use the DSA techniques of 2D CT in a 3D system.
Furthermore, the use of digital subtraction angiography in 3D imaging systems would allow the level of contrast agent used to be significantly reduced, thereby greatly reducing the stress put on the heart and kidneys of the patient by the use of the contrast agent.